Contamination of subsurface materials with hazardous compounds as a result of spills, leakage, surface discharges, and the like is a serious and pervasive problem. Liquid and liquid-borne contaminants seep into subsurface soils and/or rock formations and may ultimately percolate downwardly to contaminate the groundwater. Contaminated groundwater is typically recovered from specialized groundwater recovery wells, while vapors and gases are recovered from independent vapor recovery wells. Cleanup of contaminated subsurface materials and groundwater involves extensive treatment, frequently requiring long treatment periods and substantial resources.
Removal of contaminated groundwater from groundwater recovery wells using of one or more pumps is a known technique. A pump is typically submersed in the well and operated to withdraw water from the well until all of the groundwater that seeps into the well has been withdrawn. The contaminated groundwater may then be treated for removal of contaminants using any of a variety of known techniques.
Conventional groundwater recovery systems typically utilize devices such as float switches. In these systems, if the groundwater level increases above or decreases below a certain level, the float switch is tripped. In this fashion, the groundwater level is monitored on at least an intermittent basis. These systems do not, however, provide continuous, accurate information as to the groundwater level within the well. Moreover, they do not provide any mechanism for preferentially recovering groundwater from selected levels within the recovery well.
Recovery rates and efficiencies for groundwater recovery wells vary depending upon placement, subsurface conditions, and the like. The shape of the radius of influence for a particular well is dependent on a number of factors, including the pumping rate and duration, the hydraulic gradient, and the characteristics of the aquifer. Discharge from a pumped well is initially derived from storage within the wellbore and the aquifer immediately surrounding the well. As pumping continues, the radius of influence expands and more water is removed from storage within the aquifer at greater distances from the well.
Parabolic capture zones can also be characterized for each pumped well. Substantially all groundwater within the capture zone of a pumped well is ultimately recovered from the well if the pumping duration is long enough. Groundwater and associated contaminants located outside the capture zone are not recovered. Migration of groundwater located outside the capture zone but within the radius of influence of a pumped well is impeded, but that groundwater will generally not be captured by the pumped well. The overlap between the radius of influence and the capture zone of a single pumped well or a plurality of pumped wells creates local changes in the water table around the pumped well(s) over time, which can be illustrated using water table contour maps.
The downgradient area influenced by the capture zone of a pumped well is generally much smaller than the upgradient reach. The size of the capture zone, unlike the radius of influence, does not change over time. Consequently, the effectiveness of the groundwater recovery component of remediation programs depends, at least in part, upon proper placement of the recovery well(s).
Recovery and treatment of groundwater alone rarely provides a satisfactory level of contaminant removal, since significant subsurface contamination is present in gaseous and vaporous forms. This is particularly true of contaminants such as volatile organic components (VOCs) and semi-volatile organic components (semi-VOCs) that have relatively high vapor pressures and relatively low solubilities in water. VOC contaminants include, for example, methylene chloride, 1,1-dichloroethylene, 1,1-dichloroethane, trans-1,2-dichloroethylene, 1,2-dichloroethane, 1,1,1-trichloroethane, 1,2-dichloropropane, trichloroethylene, tetrachloroethylene, benzene, ethylbenzene, toluene, xylenes, and the like. Typically, separate vapor recovery wells are drilled and gases/vapors are withdrawn from separate vapor recovery wells using different recovery techniques.
Methods for removing contaminants from soils and other subsurface materials may require excavation and removal of contaminated materials from the site. Another technique involves leaching of contaminants from subsurface materials by percolating water through the contaminated soil. The contaminated percolated water is then withdrawn after it has reached the water table. Existing methods typically require long treatment periods and substantial expense, and they are not generally effective to remove contaminants having a low water solubility.
U.S. Pat. No. 4,593,760 discloses recovery techniques for removing volatile contaminants from the vadose zone between the surface of the earth and the undergroundwater table. According to the teachings of the '760 patent, the vapor recovery well comprises a lower perforated portion that permits fluid flow from the surrounding subsurface material and an upper impervious portion that inhibits leakage of air from the surface of the ground. When a vacuum is applied, vapor contaminants in the vadose zone are drawn through the perforated well casing. Once contaminants have entered the well, they are removed for treatment by means of fan, blower, vacuum pump, or the like. The well is constructed so that fluids in the vadose zone flow into the well, but liquid in the saturated zone below the water table cannot substantially flow into the well. Moreover, the vacuum applied is insufficient to draw a significant amount of water from the saturated zone into the wellbore conduit.
Eductor systems for extracting water from wells have been used in drinking water well applications. Eductor systems extract water from a well by pumping water through a constricting orifice and then through an expansion nozzle to create a vacuum. Individual pumps are typically provided for each individual well, and the volume of water extracted is modulated by alternately activating and deactivating the pump/eductor system. The inventor is not aware of previous uses of eductor systems in contaminated groundwater recovery systems.